In a small bowl, whisk together soy sauce, rice vinegar, brown sugar, and cornstarch. Set aside.
Heat sesame oil in a large skillet or wok over medium-high heat. Add pork slices and cook until browned. Remove and set aside.
Add vegetable oil to the pan. Stir-fry bell peppers, zucchini, and red onion for 3-4 minutes until tender-crisp.
Add garlic and ginger, cooking for another 30 seconds until fragrant.
Return pork to the pan. Pour in the sauce and stir until everything is well coated and sauce thickens.
Serve hot over rice, garnished with sesame seeds and green onions.